The present invention relates generally to an antenna system with broad-band operating characteristics and, more particularly, to a quadrifilar helix antenna for use in the Sirius Satellite Radio (2320-2332.5 MHz), XM Satellite Radio (2332.5-2345 MHz) and the like.
An active quadrifilar helix (QFH) antenna is currently used in mobile satellite communication. QFH antennas are known in the art. As disclosed in xe2x80x9cFixed and Mobile Terminal Antennasxe2x80x9d(by A. Kumar, Artech House, 1991, Chapter 5, pp.163-174), a QFH antenna comprises four helices, circumferentially and equally spaced on a dielectric cylinder or some dielectric disk support and fed with equal amplitude signals driven in phase quadrature. As shown in FIG. 1, the antenna requires a phasing network or balun, which connects to the four helices for providing signals having a 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0 phase relationship to the helices and for matching the impedance of the helices to a coaxial feed line. The quadrifilar helix can be fed from the bottom, as shown in FIG. 1. Currently, the phasing network for feeding the helices incorporates multiple 90xc2x0 hybrids, as shown in xe2x80x9cModified Quadrifilar Helix Antennas for Mobile Satellite Communicationxe2x80x9d (1998 IEEE AP-S Conference on Antennas and Propagation for Wireless Communications, pp.141-144). A number of such hybrids are commercially available in both discrete form and single chip form. In the single chip form, there are four outputs extended from the chip for providing electrical connections to the helices. The insertion loss of the feed circuit incorporating the single chip is typically in the 0.75 to 1.25 dB range. Similar insertion loss is also found on the discrete hybrids. This level of insertion loss is unacceptable for use in either the Sirius or the XM systems.
Alternatively, the quadrifilar helix can be constructed as two orthogonally arranged bifilar helical antennae to be fed from the top, as shown in FIG. 2 and disclosed in xe2x80x9cFixed and Mobile Terminal Antennasxe2x80x9d(by A. Kumar, Artech House, 1991, Chapter 5, p.168). As shown, the helix is fed from the top by running two coaxial cables to the lower end of the helices so that the bifilar antennae can be phased by a single hybrid. The high insertion loss, in this case, is mostly due to the length of the coaxial cables. Such an quadrifilar antenna is also unacceptable for use in the Sirius and XM systems.
It is, therefore, desirable to provide a phasing network, wherein the insertion loss can be reduced so that they can be used with the Sirius, XM and similar systems.
It is an object of the present invention to provide a radio antenna operable in the frequency range of Sirius and XM systems and the like, wherein in the insertion loss is greatly reduced.
It is another object of the present invention to provide a radio antenna system based on the known quadrifilar helix which is fed from baluns, wherein the baluns are simple and costeffective.
Thus, the first aspect of the present invention is a method of feeding a quadrifilar antenna having four helical arms circumferentially and equally spaced on a dielectric cylinder, wherein the arms have electrically connected first ends and separated second ends located on different quadrants of a circle. The method comprises the steps of:
providing a first balun having two feed points located on the opposite quadrants of the circle for feeding two of the helical arms, and
providing a second balun having two feed points located on the different opposite quadrants of the circle for feeding the other two of the helical arms.
Preferably, the first balun comprises:
a dielectric substrate having a first side and an opposing second side;
two electrically conductive planes located on the first side for separately providing the two feed points of the first balun; and
a first feed line located on the second side for electromagnetically coupling the electrically conductive planes of the first balun for providing a 180xc2x0 phase differential between the two feed points of the first balun, and
the second balun comprises:
a dielectric substrate having a first side and an opposing second side;
two electrically conductive planes located on the first side for separately providing the two feed points of the second balun; and
a second feed line located on the second side for electromagnetically coupling the electrically conductive planes of the second balun for providing a 180xc2x0 phase differential between the two feed points of the second balun.
Preferably, the method also comprising the step of combining the first and second feed lines at a common feeding point on a combiner, wherein the combiner has means for providing a 90xc2x0 phase differential between the first and second feed lines.
The second aspect of the present invention is a radio antenna system based on a quadrifilar antenna having four helical arms circumferentially and equally spaced on a dielectric cylinder, wherein the arms have electrically connected first ends and separated second ends located on different quadrants of a circle. The antenna system comprises:
a first balun having two feed points located on opposite quadrants of the circle for feeding two of the helical arms, and
a second balun, orthogonally arranged relative to the first balun, wherein the second balun has two feed points located on different opposite quadrants of the circle for feeding the other two of the helical arms.
Preferably, the first balun comprises:
a dielectric substrate having a first side and an opposing second side;
two electrically conductive planes located on the first side for separately providing the two feed points of the first balun; and
a first feed line located on the second side for electromagnetically coupling the electrically conductive planes of the first balun for providing a 180xc2x0 phase differential between the two feed points of the first balun, and
the second balun comprises:
a dielectric substrate having a first side and an opposing second side;
two electrically conductive planes located on the first side for separately providing the two feed points of the second balun; and
a second feed line located on the second side for electromagnetically coupling the electrically conductive planes of the second balun for providing a 180xc2x0 phase differential between the two feed points of the second balun.
Preferably, the antenna system also comprises a single combiner for electrically connecting the first feed line and the second feed line at a common feed point, wherein the single combiner has means for providing a 90xc2x0 phase differential between the first and second feed lines.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 3 to 6b.